In most mechanical watches, the energy necessary for the rotation of the hands (for example hands indicating minutes and hours) is accumulated then dispensed by a balance-hairspring system, which comprises an inertia flywheel called balance, associated with a spring in the form of a spirally-wound tape, called hairspring.
By an internal end, the hairspring is fixed onto an axis secured in rotation to the balance; by an outer end, the hairspring is fixed onto a balance-spring stud mounted on a balance-spring stud-holder which is itself secured to a fixed bridge (or cock).
Traditionally, the hairspring is made from a steel alloy based on cobalt, nickel and chrome. A quenching and an annealing are commonly applied to this alloy, which has the advantage of conferring upon it a high limit of elasticity and therefore a good breaking strength. Another advantage of the steel is its suitability for repair. However, a drawback of the steel is its magnetizable nature, which is detrimental to behaviour under load (and therefore to the accuracy of the timepiece movement); in addition, the fixing of the balance-spring stud by gluing is difficult on steel.
It is also known practice (although less commonplace) to use, for the production of the hairspring, silicon. According to Vermot et al (Traité de Construction Horlogère, Presses Techniques et Universitaires Romandes, 2014, pp. 712-713), silicon has the advantage of exhibiting a low moment of inertia, a low expansion coefficient, a good corrosion resistance and of being amagnetic. In addition, it is possible to fix the balance-spring stud at the outer end of the silicon spring by means of a two-component glue activated by ultraviolet radiation, which offers a very high fixing power. However, the main drawback with silicon is its breakable nature, in the conditions described hereinbelow.
The rotation of the balance is maintained—and its oscillations counted—by an escapement mechanism comprising a pallet assembly driven by an oscillating movement of low amplitude, provided with two pallets which drive the teeth of an escapement wheel. Thus driven, the escapement wheel has imposed on it a stepwise rotational movement whose frequency is determined by the frequency of oscillation of the pallet assembly, which is itself locked onto the frequency of oscillation of the balance (that is to say of the hairspring).
In a traditional escapement mechanism, the frequency of oscillation is approximately 4 Hz, or approximately 28 800 alternations per hour (Ah). One objective of the good watchmakers is to ensure the isochronism and the regularity of the oscillations (or constancy of the rate) of the balance.
It is known practice to set the rate of the balance by adjusting the active length of the hairspring, defined as the curvilinear length between its inner end and a counting point, located in the vicinity of the outer end of the hairspring and generally defined by a pair of abutments borne by a key mounted on a regulator.
In operation, this regulator is fixed in rotation relative to the axis of the hairspring. However, it is possible, by a manual intervention, to finely set the angular position thereof, for example by pivoting, by means of a screwdriver, an eccentric acting on the regulator in the way of a cam.
The assembly comprising the bridge, the regulator, the key, the balance-spring stud-holder, the balance-spring stud, the axis, the spring and the balance, is commonly called “adjustment mechanism”. Examples of adjustment mechanisms are proposed by the international application WO 2016/192957 and by the European patent EP 2 876 504, both in the name of timepiece manufacturer ETA.
Some interventions on the adjustment mechanism can require the unwinding (even the complete dismantling) of the hairspring. The balance-spring stud, secured to the outer end of the hairspring, then has to be separated from the balance-spring stud-holder.
This operation, called de-pegging, is difficult. The watchmaker generally holds the balance-spring stud by means of a pair of tweezers, then delicately removes the balance-spring stud.
However, it often happens that the balance-spring stud escapes from the tweezers, which provokes an abrupt release of the hairspring whose outer end is thus freed.
This incident is inconsequential when the hairspring is made of steel (and more specifically of steel alloy, as indicated above), because the quenching and tempering treatments which are applied to it make it sufficiently ductile to allow for a rewinding of the hairspring.
On the other and, that same incident for a hairspring made of silicon which, statistically, breaks in more than one case in every two, is dramatic.
The objective of the invention is to allow for a de-pegging by limiting, even eliminating, the risk of breaking.